The present invention relates to a switching power-supply unit with a DC-voltage input, which unit contains a pulse-width modulator controlled by the output voltage and at least one switch, actuated by the pulse-width modulator, and, at its output side, a low-pass filter including of an inductance and a capacitance. In this arrangement, a DC-voltage applied to the input is chopped by the switch and the AC-voltage generated in this manner is smoothed out by the low-pass filter at the output. The pulse-width modulator is controlled by feedback in such a manner that the output voltage approximately maintains a pre-set rated value.
Such switching power-supply units are used in order to efficiently generate a constant DC-voltage which is suitable, for example for powering of electronic units from a possibly fluctuating DC-voltage source.
A switching power-supply unit (Buck-type regulator) of this general type is known (International Semiconductor Databook 1981, pp. 731-733 by Unitrode Corporation or U. Tietze, Ch. Schenk: Halbleiter-Schaltungstechnik, 5th edition, pp. 391-394, Springer-Verlag 1980), in which a switch actuated by the pulse-width modulator is located between the input and the low-pass filter, and the input of the low-pass filter is connected to zero, for example ground, through a free-wheeling diode.
Furthermore, a switching power-supply unit of this type is known (International Semiconductor Databook 1981, pp. 733, 734) which contains a transformer having a symmetrical primary and secondary winding, the center tap of the primary winding being connected to a DC-voltage input and the outer connections each being connected by a switch, actuated by the pulse-width modulator, to ground and the secondary winding, in conjunction with two diodes, forming a full-wave rectifier the output of which is connected to the input of the low-pass filter.
In such switching power-supply units, the inductance of the low-pass filter limits the rate of rise of the current to a maximum value permissible for the correct operation of the circuit. If an input voltage is applied to the de-energized supply unit, the current through the inductance rises only slowly and the capacitance of the low-pass filter is charged at a correspondingly slow rate; thus there is a considerably delay before the output voltage reaches its rated value. The lower the frequency of the chopper and the greater the dynamic range of the load current which has to be covered with the prescribed constant value of output voltage and the larger consequently the values of the components of the low-pass filter, the greater is this delay.
Further symmetric chopper circuits are known which work with a constant duty-cycle and where only a capacitor is connected to the output of the full-wave rectifier for smoothing the output voltage. These choppers do not exhibit a voltage-regulator effect. The output voltage is proportional to the input voltage, the proportionality constant approximately equal to the ratio between the number of secondary turns to the number of primary turns on the transformer. When the input voltage is switched on, the capacitor at the output is rapidly charged; the rate of charging is determined only by ohmic resistances of the transformer windings and of the electronic switch in conjunction with the dimensioning of the capacitor.
None of these circuits are suitable for applications, where, apart from the voltage-regulator effect, it is also required that the rated value of the output voltage should be reached as rapidly as possible after the input voltage has been applied.
One case of application in which such requirements occur is, for example, that of supplying units for monitoring a power-line (for example instrument transformers) or auxiliary devices of such units powered from the current of the line to be monitored.